They should be reading the fucking source code. Redux isn't big. But they are stuck in a rut.

IMHO, from the outside, senior developer rut looks like a fear/resistance to change or learning. I'm not sure if what is really going on inside, though. Probably career burnout.

I am the around same age as the guys I'm referring to here, fwiw. So, it's more than just physiological aging.

Out of curiosity, what was their primary technology background before being expected to use/learn React?

Javascript :(